Heretofore, an acceleration sensor system used for an air bag system is generally comprised of an acceleration sensor unit having a piezoelectric element provided with electrodes on opposite faces and generating a collision signal upon the collision of a vehicle, and an amplifier circuit amplifying the collision signal generated from this unit. In this prior art, impact upon the collision of a vehicle is detected by an acceleration sensor unit and the collision signal is amplified by an amplifier circuit and fed to air bag system for actuating it.
However, in the prior art, it is necessary to check its function even if a highly reliable acceleration sensor unit is employed because it concerns human life.